How can I extend the unit selection logic in my RTS game to apply to multiple units?

Update:
I've now made classes and objects to hold the unit positions, and I can blit several to the screen and move them all at once. However what I haven't been able to do is get the units to be selected independently. I have these lists/arrays which draw from classes:

The selected units would then be drawn from the list 'selected', however, I am getting an error:

The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

------------------Original question----------------------

Currently if you left-click on the unit, it becomes 'selected' (or 'de-selected'), and a green square is drawn around it. Then when you right-click somewhere on the screen, the unit moves neatly into the square in the location that you clicked.

Also if you use the up, down, left or right keys it will scroll the screen.

# --- If left click pressed and the curser was on a character, select that character
if button_type[0] == 1 and mouse_position[0] >= character_location_x and mouse_position[0] <= character_location_x + character_width and mouse_position[1] >= character_location_y and mouse_position[1] <= character_location_y + character_height:
print("Unit selected",unit_selected)
print(button_type)
unit_selected += 1
unit_selected /= unit_selected #(Otherwise it will add up unit selected if you click more than once)
int(unit_selected)

# --- If right click pressed and a character was selected (and it's within the game screen), move the character to the location
elif button_type[2] == 1 and unit_selected == 1 and mouse_position[0] > 175:
mouse_position_x *= 0
mouse_position_y *= 0